In a photolithographic process used in the fabrication of semiconductor devices, such as integrated circuits, masks are used to transfer patterns to semiconductor substrates. Masks can be illuminated with wide variety of radiation sources. For example, visible, ultraviolet, and deep ultraviolet illumination sources can be employed. Typical masks comprise a transparent mask substrate, and a means to form a pattern on the mask. Mask substrates made of such as glass or quartz have good dimensional stability and transmission properties for the wavelengths of various exposing radiations. Traditionally, a layer of chrome containing material that blocks radiation from transmitting to a semiconductor substrate is formed on the transparent mask substrate to generate a pattern which is transferred to a semiconductor substrate during the photolithographic process.
In an attenuated phase shifting mask, a layer of phase shifting material that can also reduce transmitting radiation such as molybdenum silicide is deposited on a mask substrate to form a pattern. In a chromeless phase shifting mask, a pattern is formed by etching the mask substrate or adding a transparent phase shifting material to create regions where transmitted radiations have a phase shift of 180 degrees from radiations transmitted through the mask substrate.
One ultimate goal of masks is to accurately transfer a predetermined pattern to semiconductor substrates. To achieve the goal, the predetermined pattern must be first formed on a mask. Regardless of different means employed to form a pattern on a mask substrate, mask manufacturing technology primarily typically comprises forming a photoresist layer on a mask substrate, patterning the photoresist layer by a radiation source, and transferring the pattern on the photoresist layer to a means of forming a pattern on the mask. For a traditional chrome mask, the pattern on the photoresist layer is transferred to the chrome layer that blocks incident radiation to form the pattern. For an attenuated phase shifting mask, the pattern on the photoresist layer is transferred to a layer of phase shifting material that shifts the phase of incident radiation 180 degrees and reduces its intensity. For a chromeless phase shifting material, the pattern on the photoresist layer is transferred to either a mask substrate by etching or to a layer of transparent phase shifting material that reverses the phase of incident radiation 180 degrees.
Various exposing resources such as a laser or an electronic beam can be used to manufacture a mask. When an electronic beam is employed to pattern a photoresist layer, a charging effect distorts the predetermined pattern to be formed on the mask substrate because a mask substrate comprises non-conductive materials such as glass and quartz. To solve the problem, a layer of conducting material is added under a photoresist layer and above the mask substrate to absorb electrons hitting and transmitting the photoresist layer. However, due to some manufacturing reasons, a conducting layer does not fully cover the surface of a mask substrate. Thus, a charging effect occurs around the outer region of the mask substrate. The outer region comprises the non-conductive region and an adjacent portion of the conductive layer which is also adversely affected by the charging effect. Patterns on the outer region are then distorted.
As shown in FIG. 1, a conventional mask 100 has a mask substrate 110 covered by a layer of chrome 120 and leaving a non-conductive region 130 uncovered. Turning to FIG. 2A, a cross-sectional view of the conventional mask 100, a photoresist layer 230 is formed on a chrome layer 220 and a mask substrate 210. The substrate 210 comprising glass or quartz is not conductive to electrons. Electron beams 240 are used to pattern the photoresist layer 230. Some electrons are absorbed by the photoresist layer 230 and change its soluability. Other electrons arrive and hit the surface of the chrome layer 220 and the substrate 210. Electrons 260 hitting the surface of the chrome layer 220 are conducted away to a ground. Electrons 250 hitting the non-conductive region of the substrate 210 accumulate on the surface and cause charging effect because the chrome layer 230 does not entirely cover the substrate 210. As shown in FIG. 2B, the accumulated electrons 250 cause an electron beam 270 incident to the surrounding area to drift and deviate away from its original path. Consequently, the predetermined pattern is distorted.